Can multiple video cards benefit to Cycles Preview?
Just got chance to receive first-hand answer from Cycles developer:


  Yes, multiple GPUs will also help in 3D View, no extra settings or so needed. It works already.
  
  Actually, in 3D View, it does the same as regular render (by pressing F12)
  if "Progressive Refine" is enabled, which renders all tiles with 1 sample, then again, all tiles with another sample.
  
  One thing should be mentioned is that although GPU got advangtages on speed, a lot of optimizations these days still are CPU only.